The fabrication of integrated circuits (ICs) involves the formation of features on a substrate that make up circuit components, such as transistors, resistors and capacitors. The various features of circuit components are formed using optical lithography. Optical lithography involves transferring a pattern of a reticle onto a substrate. For example, an exposure source exposes a photoresist layer through the mask, which is developed to replicate the pattern on the mask. The substrate is then etched to create features on the substrate unprotected by the mask, transferring the pattern of the reticle onto the substrate.
Due to lithography processing limitations, however, it is sometimes difficult to create openings with the desired dimension or desired critical dimension (CDD). For example, in the case of forming contact openings in a dielectric layer, the CD of the contact openings may be larger than desired or required by design requirements.
A conventional technique for forming contact openings having CDD is to employ a polymerizing etch. For example, a photoresist 397 is patterned with an opening 398 having an initial CD (CDI), as shown in FIG. 3. The initial CD, for example, corresponds to the CD created by the lithographic system (CDL). The photoresist is used as an etch mask to etch the contact opening in a dielectric layer 360 on a substrate 301. By employing a polymerizing etch, polymer 392 deposits on sidewall 388 of the opening as the dielectric layer is etched. The polymer acts as an etch mask lining the sidewall. As the etch continues, more and more polymer is deposited, reducing the size of the opening. The polymerizing etch results in an opening with a tapered sidewall. The bottom of the opening has a final CD (CDF) which is smaller than CDI or CDL.
However, such a process is difficult to control and has a narrow process margin. Insufficient polymer deposition results in an opening having a CDF which is larger than CDD, which can lead to shorting or leakage problems. On the other hand, too much polymer being deposited results in an opening having a CDF which is smaller than CDD. This may lead to incomplete filling causing high resistance or opens in the contact. In either case, reliability or functionality of the IC is negatively impacted.
From the foregoing discussion, it is desirable to provide improved CD control of contact openings or other features.